The Norwalk sisters and about 1.8 million of their fellow Girl Scouts dominate the cookie industry during the first quarter of each year, then completely disappear from it in March or April, in one of the business world's most unusual seasonal events. They're so effective that the Girl Scouts of the USA had five of the 10 best-selling cookies in the United States during the first three months of last year.

The Girl Scouts generated cookie sales of roughly $785 million in the first quarter of 2012, according to the Mintel consumer research firm, which estimates a similar showing this year. That's enough to establish the nonprofit organization as the No. 3 cookie company in the U.S.

What's the key to the girls' success?

"I just tell people they're really good," Katelyn said of her sales pitch. "They're yummy."

"Yeah, they're yummy," echoed little sister Laura.

The Norwalk sisters have sold more than 1,300 boxes so far this year - including six boxes they've eaten themselves. They sold 752 boxes during the first quarter of 2012. The LeFleurs work mostly door-to-door, with Mom in the background, and use their share of each $3.50 box sale to pay for Girl Scout summer camp.

In Iowa, about 94 cents of every box goes to the cost of the cookies and promotion. The rest goes to the regional councils and local troops, and includes summer camp expenses and other activities.

The LeFleurs are part of a national sales force that has been hitting the streets and transforming parents into workplace cookie dealers every year since 1936. The powerful combination pushed the Girl Scout's flagship flavor, the Thin Mint, past Oreos in the first quarter of 2012 to make it the No. 1 cookie in the U.S. with nearly $200 million in sales. Oreos are produced by Mondelez International, formerly known as Kraft Foods Inc., a Chicago-based food juggernaut with annual revenue of $35 billion and 100,000 employees.

For many Americans, Girl Scout cookies are an iconic product with a sentimental value that transcends many other food products. It reminds them of the days when they were Girl Scouts and when their sisters or daughters participated in the program.

"The annual Girl Scout cookie sale is a force of nature at the national level," said John Frank, a Mintel food analyst. "Big companies like Kraft know it's coming, and they've learned to live with it. It's like a storm and there's nothing they can do but wait for it to pass, because there is no upside to marketing against the Girl Scouts."

Frank said the annual event reminds him of the times when he went door-to-door selling cookies with his own daughter years ago. He's forecasting 15 percent growth in the U.S. cookie market through 2017 and said Girl Scout Cookie sales should expand at the same pace, lifting their sales past the $900 million mark.

The Girl Scout cookies are made by two different bakery companies, which sometimes use slightly different ingredients and names.

"I always buy whenever a Girl Scout comes to my door," said West Des Moines resident Kathleen Till Stange, who has purchased six boxes this year. "I want to reward them for their effort and hard work, and I like the cookies, too. They don't last long at my house."

Till Stange said her favorite Girl Scout cookie is the Peanut Butter Sandwich, and she finds it puzzling that the Thin Mint is the bestselling sweet. Now head of investor relations at FBL Financial Group, she sold the cookies in her youth to pay for Girl Scout summer camp.

Harry Balzer, a national food expert at the NPD Group research firm, said the driving force in U.S. food consumption is habit. That works to the advantage of the Girl Scouts by virtue of their 77-year-old national sales program, he said.

"Girl Scout cookies only come around once a year, and they're very much like Halloween is to candy and Thanksgiving is to turkey," Balzer said. His favorite Girl Scout cookie is the Do-Si-Do. He buys two or three boxes a year.

Analysts disagree about whether the Girl Scouts grab market share from the rest of the cookie industry or enlarge the market. Jonathan Feeney, a food industry analyst at Janney Montgomery Scott LLC, said it's probably a little bit of both.

Officially, Girl Scout Cookie season begins Jan. 1 and lasts through April 30, but the 112 regional councils across the nation each conduct their own six-week sales period. Most council sales occur in February and March. Prices vary from $3.50 to $5 a box.

The Girl Scouts of Greater Iowa, which includes the Des Moines area, serves 154,000 girls and 4,000 adult members in western Iowa, northeast Nebraska and southeast South Dakota. Its sales end March 19, which means many local girls are already done selling cookies this year.

Like any good sales team, the LeFleurs have already incorporated that impending shortage into their sales pitch.

"You can freeze them in the fridge - buy them for the year," Katelyn said. "These are a lot better than Chips Ahoy."

Girl Scouts of the USA sells more than 200 million boxes each year. The Greater Iowa council sold 1.8 million boxes last year.

Thin Mints typically claim about 25 percent of all national Girl Scout Cookie sales, followed by Samoas, Tagalongs and Do-Si-Dos. Thin Mints accounted for 23 percent of all cookie sales by the Girl Scouts of Greater Iowa.

The cookies are so popular that they've begun showing up on sales websites like eBay, even though all online sales are prohibited by Girl Scouts of the USA.

"We don't have a demand problem - people want Girl Scout cookies," said Amanda Hamaker, manager of product sales for the Girl Scouts of the USA.

The challenge is sometimes too much parental involvement and reminding people that Girl Scouts is not a cookie company, she said. The girls are supposed to be involved in all the selling that takes place.

The purpose of the annual cookie sale is to teach girls about goal setting, decision making, money management, people skills and business ethics. The girls receive merit badges for each skill.

The lessons are a bit more fundamental for tiny Laura LeFleur. Her mother, who serves as the Norwalk troop leader, said the cookie sale is a great way to teach older girls about business. It's also helping her youngest come out of her shell and improve her math skills.

"My mom has to do the change because I'm not very good with change yet," Laura said shyly.